Miocene

The Tertiary deposits of the western United States have yielded a remarkably complete story of the history of land mammals throughout the entire extent of the Age of Mammals. Although the Pleistocene, and the last phase of the Tertiary, are well represented in the eastern United States and a few marine deposits of Miocene age are known, only one early terrestrial deposit of any consequence is present in the known sedimentary rocks east of the Mississippi River. The reason for this lack of a fossil record, in this part of North America, is due to the early Tertiary sediments being dominantly marine in nature and hence containing no land mammals. The one exception to this barren record lies in north central Florida. This deposit, the richest bone bed of Miocene age in eastern North America, is located in Gilchrist County in a most unpromising-appearing setting of low, sandy flat woods having none of the "usual 11 surface outcrops visible with which vertebrate fossils are associated. The circumstances that led to the discovery, purchase and development of the now famous Thomas Farm quarry are worthy of relating here in some detail.

In September 1931, Mr. J. Clarence Simpson, of the Florida Geological Survey, was investigating a reported Indian graveyard that had turned up while plowing through a depression in an abandoned field of the old Raeford Thomas Farm located between Bell and Ft. White. Mr. Clarence Simpson determined correctly that these bones were not of human origin but represented, instead, the remains of the small three-toed horse Parahippus and were similar to those obtained from the fuller's earth pit at Midway, Florida, in Gadsden County. A small collection of fragments from those that littered the surface of the shallow depression which marked the original site, were sent back to the Geological Survey office in Tallahassee. The Survey Director at that time, Dr. Herman Gunter, forwarded these scraps to Dr. G. G. Simpson at the American Museum of Natural History in New York City. Dr. Simpson, a recognized authority on fossil mammals, of course recognized the scientific importance of this find and urged that more material be collected if possible.

Dr. Gunter secured permission to excavate and several more trips were made to the farm by personnel of the Florida Geological Survey between 1931 and 1932. A published account of the first material obtained at this dig was released by the Florida Survey in 1932 (Simpson, G. G. , Miocene Land Mammals from Florida, Florida Geol. Survey Bull. 10, 58 p.).

Plate VI. Miocene horse Parahippus and dog-like carnivore Tomarctus.

In 1939, Dr. Thomas Barbour, Director of the Museum of Comparative Zoology at Harvard College, made one of his frequent trips to the Sunshine State to obtain fossils for the Harvard Museum and stopped for a visit at the office of the Florida Geological Survey. During the course of his stay in Tallahassee, Barbour had occasion to examine the fossils that had been obtained from the newly-opened deposits at the Thomas Farm.

The result of this visit was a desire, on Barbour's part, to purchase the forty acres of land that contained the fossil quarry so that it would be protected for future scientific excavations. Dr. Gunter located the owners, a loan and trust company in Georgia, and undertook the initial negotiations for the purchase of the desired land. The property was purchased and deeded over to the present owner, the University of Florida, with the verbal understanding that Harvard University and the Florida Geological Survey would also enjoy the privilege of collecting fossils from the Thomas Farm quarry, for scientific study or display. The Florida Geological Survey has received the cooperation of both universities in its endeavor to obtain a series of vertebrates from this locality for the state collections that are housed in the Survey's present quarters in the State capitol at Tallahassee.

The nature of this locality, as it appeared in Miocene time, has not been solved to the satisfaction of all concerned. Indications point to a partially filled sinkhole or to a cavern or rock shelter having considerable depth, located perhaps at the edge of a stream. That a cavern of some sort was present is attested to by the numerous bat remains that are found in the rubble of a boulder bar or breakdown of a long collapsed cave roof. That this cavity was at times water fed is indicated by the various amphibian, aquatic turtle and alligator remains that are present in the sediments. However, no reliable or identifiable fish bones have turned up in the nearly three decades of digging since the quarry was first discovered. Another indication that this deposit was stream fed at one time or another, while the animals were being entombed, is substantiated by the water worn scraps of bone and by the evidence that no articulated or individually associated skeletons have been found. Instead, it is not unusual to find five or six horse skulls nesting together or half a dozen or so femora, of the same side of the animals represented, lying in close contact. Although quite a few complete skeletons are known of the small horse Parahippus (pl. VI), the different elements composing these complete skeletons many represent several individuals rather than belonging to one animal as is usually the case in most vertebrate fossil quarries from which complete mammal skeletons are known.

Among the animals, represented in the known collections from this site, are the remains of the large bear-like carnivore Amphicyon, which rivaled the Kodiak bear in bulk and in having a similar battery of powerful teeth. Also present are the smaller coyote-sized dogs Cynodesmus and Tomarctus (pl. VI), as well as a badger Leptarctus and a small skunk Miomustela. A few long-snouted camels known as Floridatragulus as well as the small dik-dik sized artiodactyl Blastomeryx were also dwellers of the Thomas Farm area in Miocene time. The remains of two different sized horn less rhinoceros have occasionally turned up in the excavations.

One of the interesting things concerning this Florida locality, as compared with those of similar age found in the western United States (text fig. 7), is the total lack of the remains of either felids or Oreodonts. Both of these groups of animals are well represented in similar quarries throughout the western United States and the latter animals are so numerous in some areas that certain layers that contain their bones have been dubbed "Oreodonbeds" by the paleontologists that work these beds. No positive statements can be made, based on our present knowledge of these forms, as to why they would occur in great abundance in one area and be totally absent in another.

The present limits of the excavation, that contain the most productive collecting area, measures approximately 30 by 60 feet and reaches a depth of 15 feet below the surrounding terrain. Test borings made by the Florida Geological Survey indicate that the bone -bearing beds extend to a depth of about 30 feet below the present bottom of the pit and become barren of bone about 100 feet out from the present center of operations.

This quarry has actively been worked by one party or another from each of the three institutions concerned since 1941. Dr. A. S. Romer of Harvard University, and present Director of the Museum of Comparative Zoology, has postulated that the pit would not be completely excavated until approximately 2000 man-years of labor had been expended.

It must be stressed that anyone contemplating visiting the Thomas Farm quarry will have to have written permission from the head of the Biology Department of the University of Florida. This precaution is to prevent uncontrolled wandering over the bone deposit, which would destroy scientific material that could not be replaced.

The Griscom Plantation, or Luna Plantation, as it is generally known today, is located about 15 miles north of Tallahassee in Leon County. This plantation is the site of an early Miocene vertebrate locality that was accidentally discovered in 1916 during the course of digging a shaft for a water well. This shaft, having a diameter of six or eight feet, was dug to a depth of 50 feet before it had to be abandoned due to encountering poisonous gases. The workmen had struck a bone-bearing layer, just before the pit was vacated, which has produced the types of the Miocene horse Parahippus leonensis and the dog-like Gynodesmus iamonensis. The well was completed by the use of a mobile drill rig and the larger hand dug opening was filled in around the well casing, no additional bone fragments being collected. This bone-bearing layer does not outcrop on the surface in the vicinity of the plantation and, since the original well is now in the landscaped area of the plantation headquarters, it is improbable in the foreseeable future that additional material will be collected f from this locality. All of the animals obtained f from this well, with the exception of the carnivore Temnocyon, are also known f from the Thomas Farm quarry. This last named form has been recorded from a small bone -bearing pocket situated in Pit No. 2 of the Franklin Phosphate Company's mine near Newberry in Alachua County. This pit is now abandoned and a good deal of the exposures are covered with redeposited surface soil or vegetation so that the possibilities of getting good additional material from this locality are poor indeed.

As in the case of the Griscom Plantation, the digging of apumppitby the Tallahassee waterworks was responsible for some very tantalizing fragments of the Miocene rhinoceros Aphelops and a camel Nothokemus. These meager scraps were collected in 1930 and here again, as in the Griscom Plantation locality, the bone-bearing layer is no longer available for further exploration.

The most recent locality of Miocene age to come to light was exposed by a road cut through Colclough Hill, south of Gainesville in Alachua County. This layer, judging by the fauna, was laid down as a marine or brackish water deposit. The animals from this layer have been identified as the small Miocene horse Parahippus blackbergi, a squirrel like rodent and numerous shark and ray teeth. Although this site will most surely never be developed as a quarry, enough material has been collected as surface scrap to warrant future investigation, particularly after heavy rains.

Only two good Miocene localities have been reported from the Florida panhandle. Both of these are located in Gadsden County and were located in the fuller Is earth mines of this area. The first of these localities, at Quincy, produced Florida's first identifiable material of the Miocene horse Merychippus. From the second locality, at Midway, were recorded scraps of Parahippus and Nothokemus as well as Merychippus. Both of these sites are now in abandoned water-filled pits. The surrounding country is covered by brush so that little hope is held for any additional fossils being obtained from the original locations.

The fossil bed so Polk County will be discussed in some detail in the following section on the Pliocene beds of the Bone Valley, but a note is in order in this section pointing out that vertebrate remains, found only in the Miocene in other parts of the western hemisphere, have been taken up from the bone Valley deposits.

These "true" Miocene forms are the badger Leptarctus, the tapir Tapiravus, and the cetacean Hoplocetus.

Text figure 7. Age correlation chart of Florida Miocene with that of North American provincial stages.

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